Is Pluto still not a planet again?

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Pluto might not be a planet anymore, according to the definition of planethood adopted by the International Astronomical Union in 2006, but the icy little orb on the edge of the solar system still has a lot going on. Today NASA’s Hubble telescope has discovered a 4th moon around the new non-planet. The space telescope was searching for rings around the planetary oddball at the edge of our solar system when it came across P4, the temporary name for the newly discovered moon. With an estimated diameter of 8 to 21 miles, P4 is the smallest of Pluto’s four moons, the U.S. space agency said in a statement.

Pluto’s largest moon, Charon, is 648 miles across, and its other moons, Nix and Hydra, are in the range of 20 to 70 miles in diameter. P4 was found orbiting between Nix and Hydra which were both discovered by the Hubble telescope in 2005. Charon was discovered in 1978 at the US Naval Observatory.

Despite the fourth moon’s tiny size, it is not the smallest moon in the solar system. The current title goes to two moons orbiting Jupiter, J9 and J12, which are both believed to be less than one mile across. Several smaller “moonlets” have also been discovered orbiting Saturn. The observation by Hubble is part of ongoing work to support NASA’s New Horizons mission, scheduled to have a close encounter with Pluto and its moons in 2015. Hubble observation programme leader Mark Showalter of the SETI Institute in Mountain View, California, said the discovery of such a tiny fourth moon was “remarkable”. He said: “I find it remarkable that Hubble’s cameras enabled us to see such a tiny object so clearly from a distance of more than three billion miles.” P4 was first seen in a photo taken by Hubble on June 28 and was confirmed in subsequent Hubble pictures taken July 3 and July 18.

The race is now on for the moon to be given an official name and, perhaps ironically, the organisation given the task of finding a name is the same body that stripped Pluto of its planet status five years ago. Pluto used to be the ninth planet in the solar system but was declassified by the International Astronomical Union in August 2006 and instead given “dwarf” status. At around 1,430 miles across, the dwarf planet is approximately two-thirds the size of the moon orbiting Earth and less than one per cent of Earth’s mass.

Traditionally, new objects in the solar system are named after Greek or Roman mythology. Pluto is named after the Roman god of the underworld after a suggestion by an 11-year-old girl from Oxford called Venetia Burney; and Charon is named after an underworld character who carried the souls of the dead across the river Styx, the river between the living and the dead. Pluto’s other two moons Nix and Hydra also have names descended from Roman mythology. Nix was a goddess of the night and the mother of Charon whereas Hydra was a monster with many heads. All four of Pluto’s moons are believed to have formed when Pluto and another planet-sized body collided. Earth’s Moon may have formed the same way.

Pluto has a very thin atmosphere of nitrogen, methane and a few other gases which is only able to exist due to a “vapour-pressure” equilibrium. Pluto’s atmosphere is very sensitive to changes in surface temperature and since 1989 the dwarf planet has been moving further and further away from the Sun, according to astronomers at the Lowell Observatory who are currently observing the phenomenon.